How Long Do Cosmos Bloom? (Cosmos Guide To Flowering)

Colorful and full of life, cosmos are a great addition to a cottage flower garden and are one of the easiest flowers to grow. Today we are chatting all about the bloom time of cosmos flowers. We will also explore how you can keep your colorful flowers blooming all summer long!

How Long Do Cosmos Flowers Bloom?

Cosmos bloom for a time span of about 3.5 to 4 months. When grown from seed, these delicate flowers bloom from mid summer until late fall, which in many climates is mid July to October.

Cosmos flowers take about 7-8 weeks before they mature enough to bloom. With proper deadheading they will then produce flowers all through late summer until the first frost of Autumn.

These beautiful blooming beauties are generous in their flower production and will thrive if you give them the right conditions.

While cosmos flowers bloom prolifically from mid summer until late fall, you can get a head start on growing cosmos by growing them indoors from seeds or purchasing starts from garden centers.

Cosmos sulpher is often grown as a wild flower to attract bees and butterflies

Cosmos are annual flowering plants that are easy to grow and give an overabundance of beautiful, bright cheerful blooms. We absolutely love growing them, and have found them to be an easy plant for children to grow as well.

Deadheading Cosmos For More Blooms

Cosmos with their delicate, colorful flowers, bloom prolifically. These annuals are a cut and come again flower. This means if you dead head the cosmos plants to remove spent flowers, more will bloom!

The more you harvest cosmos, whether for bouquets or removing dead flowers, the more flowers you will get.

When cosmos are routinely deadheaded, beautiful NEW flowers will continue to bloom until the first frost. These colorful flowers are often planted alongside garden beds because they attract pollinators such as bees and butterflies with their many flowers and long bloom time.

pink cosmos bipinnatus flower

How To Deadhead

Deadheading cosmos flowers is really very easy. Follow the dead flower head down to the first set of leaves and pinch or cut the flower here. This will encourage the plant to send out new side shoots.

Cosmos are a great flower to grow with kids. Teach them to sow seeds, deadhead, and gift beautiful bouquets to friends and family!

Do not cut your flower head directly off at the top of the stem. The plant will not branch out and create new flowers if you do so.

The best time to deadhead is in the morning. Avoid removing spent flowers during the hottest parts of the day.

Learn how to grow beautiful dwarf sunflowers here.

How Long Does It Take Cosmos To Bloom?

Depending on the variety grown, it takes about 8-12 weeks for a cosmos plant to develop from seed, grow, and then produce it’s first flower blooms.

From the time cosmos seeds are planted until they produce their delicate flowers almost 3 months will go by.

You may be wondering if you can speed up the blooming process, and the answer is yes, you can!

You can purchase starts from a garden center or start your own seedlings in early spring, then transfer them to the flower bed once the last frost has passed.

pink cosmos flower up close

Types Of Cosmos Flowers

There are many types of Cosmos you can grow, from shorter varieties to towering types. They come in many colors and shapes. Cosmos originate from South and Central America, and their natural climate is dry and arid. All varieties thrive on heat, moderate soil and and little water.

Here are a few of our favorites:

Cosmos sulphureus, or cosmos sulpher, is a yellow to orange daisy-like cottage flower known to attract bees and butterflies. This beautiful cosmos is often found in wildflower mixes and can grow up to 6 feet tall, though it is more commonly found to be around 3 to 4 feet tall.

Cosmos bipinnatus are the most popular garden variety of cosmos flowers. These delicate blooms come in many different colors and are often seen in pink, red and white hues.

Chocolate Cosmos are a deep, rich maroon with a chocolate scent! These unique flowers are the only cosmos flower grown by their tubers. They are also the only variety that can be grown as a perennial in some climates. Hummingbirds love chocolate cosmos.

yellow cosmos sulpher flower

Did you know? Cosmos need well-drained soil to thrive, but other than that they are not a picky flower when it comes to soil and actually prefer poor soil. (We have even grown cosmos in clay soil before and received beautiful blooms).

Letting Cosmos Go To Seed

Towards the end of the season, you can begin to let your cosmos blooms go to seed. You can then collect these seeds for the following year or let them fall from the flower and see if any take root and grow the following spring.

Cosmos seeds will need to survive winter rains, birds, and insects in order to sprout and flower in the next season.

In dry climates, some of your cosmos seeds will likely survive the winter and thrive the following spring. In wetter areas you may find that all of your seeds rot over the stormy winter season.

A Note From the Garden: Living on the west coast of Oregon, the prolific winter rains would rot volunteer cosmos seeds. We rarely saw a volunteer cosmos flower peak it’s head up in spring. My sister, who lives on the dry side of the state, had so many cosmos flowers pop up voluntarily that she almost considered them a weed!

Are Cosmos Perennial Or Annual?

Cosmos flowers are not winter hardy and are grown yearly as an annual. They are not a perennial in North America.

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In the United States, cosmos are annual plants and are grown from seeds each year. This means that the plant will not come back the following year and needs to be re seeded.

As mentioned above, seeds that drop from the flower heads may survive winter and end up re seeding themselves the following year. This may result in new cosmos flowers that you can enjoy without the work of re planting seeds!

How Long Does It Take For Cosmos Flowers To Bloom?

It takes between 2 and 3 months for a cosmos flower to bloom, depending on the variety. When planting by seed, plan on allowing 8-12 weeks from the time the seed is planted until the plant reaches full maturity and begins blooming.

pink cosmos in blue vase
Cosmos are an absolutely stunning cutting flower!

How To Extend The Cosmos Growing Season

You can extend the cosmos growing season by continually deadheading your plants. This not only keeps plants healthy but will send you bloom after gorgeous bloom.

You can get an early start on flower blooms by starting seeds indoors using seed trays and bright light.

Instead of growing from seeds, you can also pick up starts from a garden center. These plants will yield flowers much sooner, but this is a more expensive approach than growing your own plants from seed.

Seedlings and purchased starts can be set out after all danger of frost has passed.

How Long Do Cosmos Blooms Last?

Each cosmos flower will bloom for about 4 to 7 days. In dry weather the blooms will last longer, and in a summer rainstorm the flower bloom will not last as many days.

While each flower is relatively short lived, the cosmos plant produces hundreds and hundreds of blooms. Take it from us, they are worth growing!

When Do The Flowers Die?

Cosmos flowers do not survive frost. They will bloom up until the first frost and then they will die. At this point you can remove cosmos flowers from your garden or simply let them decay until spring, and then dig up the remains of the plant and place in the compost pile.

pink cosmos flowers in a bouquet

How Long Do Cosmos Last In A Vase?

Cosmos in a vase last for about 7 to 10 days. You can extend the vase life of cosmos flowers with a few simple tips.

How To Extend Vase Life

Sweet and delicate, cosmos make great cut flowers. They pair well with easy to grow sunflowers and colorful zinnias. And with just a few plants you can enjoy beautiful cosmos bouquets all summer and fall.

Here’s how to get your flowers to last longer:

  • Cut your flowers at the right time: when the buds are firm and fresh and just about to open
  • Gently remove leaves that would be submerged under water. This will prevent rot
  • Cut your cosmos flowers in the cool hours of early morning, before any wilt has set in
  • Set the long stems directly into water when you are out in the cutting garden
  • Use flower food in order to feed and extend the life of your cut flowers
  • If you have space, set your flower bouquet in the refrigerator over night
pink cosmos in a blue vase

Why Are My Cosmos Not Blooming?

Generally an easy to grow flower, there can be a few reasons why a cosmos plant fails to flower. Use this guide to help you troubleshoot your cosmos flowers:

  • Not Enough Sun: Cosmos are a heat and sun loving plant and can’t bloom without enough light! Make sure to plant your flowers in a sunny spot that receives 6 to 8 hours of sun. If you are growing cosmos in a container, try moving them to a sunnier location to increase blooms.

  • Soil Overly Rich: Very rich, nutrient dense soil sounds great, doesn’t it? Not so much for cosmos flower blooms. When the soil is overly rich, the extra nutrients will go into producing lots of green foliage. Extra rich soil will give you abundant foliage and not so many blooms.

  • Over Watered Plants: Cosmos love soil with good drainage and actually prefer life a little on the dry side. Water young plants regularly until established. From there, let the soil dry out between watering. Cosmos are a lot like sunflowers, thriving in hot weather with dry soil and little water.

  • Fungal disease can prevent cosmos from growing and thriving. Powdery mildew is a spore that floats through the air and can attach to flowers. This fungus eventually creates a white powder that covers the plant and can cause the plant turn yellow and decay. To prevent powdery mildew, make sure to space plants about 18 inches apart and water at night so the plant has time to dry before the colder nightly temperatures set in.
over fertilized cosmos flower
An over fertilized cosmos flower can grow quite tall and have fewer blooms, as pictured above!

As a young gardener I once planted cosmos in a very rich, fertile soil and gave them a deep soak of water every day. These plants grew quite strong and lush, but I got very few blooms from them!

Learn how to grow snapdragons in pots here!


Do Cosmos Bloom All Summer?

Yes cosmos definitely will bloom all summer. The key is to keep deadhead the flowers in order to stimulate more production of blooms!

Why Are My Cosmos So Tall?

Too much fertilizer and too much water can stimulate the foliage of cosmos to grow at the expense of flowers. This can detract from the energy the plant puts into flowers. This results in tall, spindly cosmos flowers that don’t have a lot of blooms.

How Can you Make Cosmos Bushier?

Bushy plants have more stems that produce more flowers! As the cosmos flower begins to grow, pinch out the middle stem to encourage branching. This can be done with a pair of flower clippers or by simply pinching out the center of the flower with your fingers.

What Is The Best Soil For Cosmos?

Cosmos flowers thrive in moderate soil. You do not need fertile soils for this prolific bloomer, and in fact overly fertile soil can cause excess foliage to grow instead of flowers.

When Do Cosmos Stop Flowering?

Cosmos stop flowering when the first frost hits. At this point the plant dies.

Do Cosmos Grow Well In Pots?

Yes, cosmos can grow well in pots and containers! Dwarf varieties make excellent container plants and will add colorful cheer to your home. Make sure to use well draining soil and a container large enough to support the growth of the plant. Avoid taller varieties for small containers as they will become top heavy and tip over.

What Fertilizer Do Cosmos Like?

For best results, do not fertilize cosmos flowers. These flowers thrive off of soil with moderate nutrient density. Fertilizing will promote foliage instead of blooms.

How Do You Keep Cosmos Blooming?

Deadheading cosmos by removing the spent blooms will stimulate the cosmos flower to keep producing blooms. Check for spent flowers regularly, every few days during peak blooming season.

In Summary

Cosmos are a beautiful flowering annual that bloom for a particularly long time (from late spring until early frost). The individual blooms of cosmos make excellent cut flowers, and this low maintenance plant is colorful, fun and easy to grow.

For best results, plant cosmos in full sun and once established avoid excess water. Don’t use much fertilizer and deadhead the cosmos plant regularly.

With their delicate flowers and rainbow of colors, cosmos are one of the many delights of summer. We hope you enjoyed learning about cosmos bloom time with us. Please comment below your favorite variety of cosmos flowers to grow!

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